The valve is under the intake manifold at the back of the engine. You need to take off the plastic housing that has M power written on it to get to the valve. It shouldnt cause any engine running problems if the valve fails, all it does is allow fuel vapours from the charcoal canister to be drawn into the engine and burnt. The valve is part no 14 on this diagram :
http://www.realoem.com/bmw/showparts...38&hg=13&fg=15